1. Technical Field
The present invention relates to medical image composition.
2. Discussion of the Related Art
Medical image composition is the act of joining image volumes from separate views from a medical image scanner into one consistent seamless whole volume. This is done via the method of composing (also called stitching or mosaicing).
In “stitching,” an algorithm is used to fill in the voxels of the output larger volume according to the locations computed by alignment. Where only one input volume overlaps an output voxel location, data is copied directly from the input to the output. Where two or more input volumes overlap at an output voxel, program logic is used to determine how to choose between the input datapoints or blend them together. Where no input volume overlaps the output, black voxels are used to fill. FIG. 1 illustrates this concept.
More specifically, FIG. 1 is an illustration of stitching aligned volumes together by blending first and second volumes (input volumes 1 and 2) in the overlap area. For ease of illustration, only the front plane of each volume is illustrated.
The goals of blending component volumes are to limit artifacts that can occur from image volume distortion and/or patient/anatomy motion. These include: (a) limiting visible seams or creases; (b) limiting contrast variation; and (c) limiting ghosting effects due to blending.
The increase of the field strength of newer magnetic resonance imaging (MRI) machines (e.g., the Siemens Trio) provides the opportunity for improved image resolution and quality. A challenge with these systems is that the increased magnetic strength has resulted in magnifying B0 effects (e.g., a magnetic loading distortion effect). B0 effects present themselves as a localized distortion in the volume. B0 distortion can occur anywhere within the volume, but is most pronounced in the regions on the periphery of the isocenter of the volume. Much effort has been made to correct for these effects in the design of MRI installations. As an example, see J. Sled, G. Pike, Correction for B0 and B1 variations in quantitative measurements using MRI, Magnetic Resonance in Medicine, Vol. 43, No. 4, 2000, pp. 589-593.
Whole body scans of a patient can take up three to five individual volume acquisitions which are then composed together to capture a complete scan of an individual. Since B0 effects are most evident in the leading or trailing edges of a MR volume, having sufficient volume overlap and discarding the leading or trailing regions is a possible solution. However, these kinds of solutions may inevitably result in the need for more volumes, and the more volumes that are required for an individual, the more time an individual subject workflow takes, which can cut into the efficiency of a MR installation. In addition, there is no certainty where B0 effects might occur in a volume. This makes narrowing the field of view of a volume a potentially expensive and imprecise solution.
B0 effects are not the only artifacts that can interfere with successful composition. For example, patient anatomy or movement between scans may also need to be taken into account.